For about the past nearly fifty years, it has been the practice to coat textile reinforcing elements such as cords and fabric (referred to herein as "cords" for simplicity) to be used in rubber goods, with an adhesive composition comprising an "adhesive rubber latex" consisting essentially of a phenol-formaldehyde resin, in which the phenol has almost always been resorcinol. This adhesive rubber latex is referred to as an "R/F/L" dip, for its three components namely, resorcinol, formaldehyde and latex. This dependence upon an R/F/L was attributed to the peculiar effectiveness of aromatic OH groups in formation of the R/F resin, as noted quite early by H. Moult in Handbook of Adhesives, Skeist, I., editor, at pg 495, published by Reinhold Publishing Corporation, New York, 1962; and even earlier, by vander Meer, Rubber Chem. Techno., 18, 853 (1945); and by A. Greth, Anger. Chem. 51, 719 (1938).
Conventionally, a two-step process has been used, comprising dipping the cords in a first bath ("dip") of a first adhesive composition, drying and heat-setting the composition on the coated cords, then dipping the coated cords in a second dip comprising the R/F/L. Glass cords are usually precoated with an R/F/L by the manufacturer of the glass cords so that they need only be woven into fabric and used to reinforce goods such as automobile tires and hose.
Another method commonly used to prepare steel wire cord (brass-plated) for bonding to rubber is to generate the resin in situ in the vulcanized rubber matrix by incorporating therein a formaldehyde (or methylene) donor, for example hexamethylenetetramine, and a formaldehyde (or methylene) acceptor, typically resorcinol. This second method has no relevance to the preparation of textile cord, except for the remarkable fact that, despite the entirely different characteristics of textile cords, and steel wire, and glass cords, and the differences in the processes for bonding each of them to rubber, resorcinol is the common essential adhesive component.
With respect to reinforcing rubber goods with textile cords, a variety of filamentary textile materials, such as rayon, nylon, aramid and polyester fibers have been used. Polyester and aramid cords are frequently preferred because of their high strength and high modulus which are particularly advantageous in goods such as tires, hose and belts, but it has been found far more difficult to achieve adhesive bond strengths between polyester and rubber, or aramid and rubber, than between nylon and rubber.
The difficulty in bonding polyester cord to rubber is generally attributed to the presence of only hydroxyl (OH) and carboxyl (COOH) groups at the ends of the polyester molecules, while in nylon (for example) there is a relatively high frequency of amide (CONH) groups along the macromolecular chain. Aramid fibers are a special case which are not as satisfactorily coated as nylon, having instead, the adhesive characteristics of a polyester cord. Rayon and nylon are treated satisfactorily with a single step (single bath) coating of an aqueous dispersion of an RFL, and as a consequence, the process of this invention is only applicable to polyester and aramid cords, and most particularly to non-adhesive-activated ("non-AA") polyester cord.
Many adhesives and bonding systems ("dips") for synthetic linear polyester cords have been used. Most are cost-ineffective and additionally suffer from various other disadvantages such as toxicity in the case of of adhesives based on glycidyl ethers, or water-soluble phenolic condensates; and/or instability, as in the case of polyisocyanates, which has resulted in the use of water-insoluble reversibly blocked polyisocyanates (RBP) which, generally being solid, tend to precipitate in the baths in which the cord is dipcoated. An RBP is so termed because the reactive isocyanate (NCO) group is blocked against reaction at low temperature below about 400.degree. F., and then the isocyanate is regenerated when the temperature is raised, usually above 400.degree. F. but below about 500.degree. F. The temperature at which a RBP will dissociate depends mostly on the blocking moiety (or substituting group).
Treatments which utilize phenol-blocked methylene-bis-(4-phenylisocyanate), and the like are disclosed in U.S. Pat. No. 3,307,966, and the use of phenol-aldehyde blocked polyisocyanates are disclosed in U.S. Pat. No. 3,226,276, inter alia. These treatments using an RBP necessarily require the use of plural dips, as does the process of this invention, or the adhesion is unacceptable. The first dip of this invention is conventionally used in the prior art.
In a typical two-step commercial process, polyester cord is dipped in a polyepoxide-containing first bath in which solid finely ground RBP is dispersed with the aid of a dispersing agent, excess RBP removed, the RBP-coated cord is dried at about 300.degree. F., and then the dried polyester cord is heat-set at a temperature below about 500.degree. F. If the cord is aramid cord, it is dipped in a first bath containing a polyepoxide and a curing agent, but no RBP. By "polyepoxide" I refer to a water-soluble epoxide having plural epoxide groups in a molecule. In a second bath, heat-set pre-coated cord is dipped in an R/F/L, excess R/F/L is removed, the cord dried at about 300.degree. F., and heat-set at a temperature below about 500.degree. F. so as to give excellent adhesion of the R/F/L to the pre-coated cord.
Though cord which has been properly coated with the aforedescribed prior art two-step process has excellent adhesion to rubber if it is immediately embedded in it, and cured (the rubber is vulcanized), the adhesion is poorer if the pretreated cord is exposed to the atmosphere and/or to sunlight. To combat this problem, I have disclosed in U.S. Pat. No. 3,968,295, the coating of an RBP-coated cord with a R/F/L in which is mixed an acrylic resin (interpolymer) such as is disclosed in U.S. Pat. No. 3,007,887, the disclosure of which is incorporated by reference thereto as if fully set forth herein. This interpolymer (copolymer) negates the degrading effects of the atmosphere and sunlight on the pretreated cords, serving an "anti-degradative" function in the R/F/L dip without adversely affecting the cords' excellent adhesion to cured rubber.
As the data in my U.S. Pat. No. 3,968,295 indicates, the pull-out force for cord treated in a two-step process, first with RBP, then with R/F/L in which is mixed the carboxylic acid ester copolymer, is essentially the same as that of rubber in which the same R/F/L dip is used without the copolymer. I had no reason to expect that this acrylic resin might have a unique effect, on a molecular scale, if the resorcinol (R) and formaldehyde (F) components were simply left out of my prior art bath. Stated differently, it was surprising that the acrylic resin acted as the dominant adhesive, rather than as adhesion promoter if the R/F was left out of the second dip, to such an extent that it provided excellent adhesion. By "excellent adhesion" I infer that, in standard "180.degree. peel adhesion" or "H-pull" tests, the surface of the stripped or pulled out cord is substantially completely coated with rubber, which is characteristic of "cohesive failure".
To avoid the operating inconveniences of a two-step process such as solid RBP setting out in the first bath, contamination of the second bath, and the like, and in addition, to save on operating costs, it is desirable to provide a one-step process for coating polyester and aramid cord with a combination of the RBP and the R/F/L in a single bath without deleteriously affecting the properties of the coated and heat-set cord, and without destroying the useful life of the bath. I have provided such a one-step process in my copending patent application Ser. No. 420,548 in which I utilize certain acrylic resins to promote adhesion. I was thus surprised to find that the RF components could be dispensed with if the appropriate concentration of particular acrylic resins, produced as emulsions, was dispersed in a conventional vinyl pyridine rubber latex.
It should be recognized that any conventionally used diene polymer latex is itself an adhesive in the sense that it bonds well to natural or synthetic rubber, but the latex does not bond to the cords' surface. This is evident from a standard H-pull test in which the cord pulled out is clean, that is, has little or no latex rubber adhering to it. Thus, the context of the latex's function in bonding reinforcing cords to rubber, the latex, by itself, is generally recognized as having no significant adhesive function.
The effectiveness of various acrylic resins as components of adhesive compositions has been long known, but it is also well known that the effectiveness of each acrylic resin depends upon the other components of the system. For example, U.S. Pat. No. 3,407,092 teaches a two-step process for coating nylon filaments, during the first step of which process an aqueous dispersion or emulsion of an acrylic copolymer is deposited, and in the second step of which process, cords made from the filaments are coated with a conventional R/F/L dip.
Soon thereafter, U.S. Pat. No. 3,483,075 disclosed an adhesive for any tire cord which adhesive was a mixture of a copolymer of methyl acrylate, (a) a monoester of a monoethylenically unsaturated dicarboxylic acid, and a a monoethylenically unsaturated hydroxyl-group containing monomeric material, and (b) an R/F/L. Thus both references taught that, whatever the differences in the acrylic resin used may be, or the particular type of cord on which the adhesive compositions are used, the R/F/L is indispensable.